Voltage-gated sodium channels (VGSC) are present in all excitable cells including cardiac and skeletal muscle cells and central and peripheral neurons. In neuronal cells, sodium channels are responsible for amplifying sub-threshold depolarizations and generating the rapid upstroke of the action potential. As such, sodium channels are essential to the initiation and propagation of electrical signals in the nervous system. Aberrant sodium channel function is thought to underlie a variety of medical disorders (Hubner and Jentsch, Hum Mol Genet 11:2435-45, 2002) including epilepsy (Yogeeswari et al., Curr Drug Targets 5:589-602, 2004), arrhythmia (Tfelt-Hansen et al., J Cardiovasc Electrophysiol 21:107-15, 2010) myotonia (Cannon and Bean, J Clin Invest 120:80-3, 2010), and pain (Cregg et al., J Physiol 588:1897-904, 2010). Sodium channels are typically a complex of various subunits, the principle one being the pore-forming alpha-subunit, which is alone sufficient for function.
Nine known members of the family of voltage-gated sodium channel (VGSC) alpha subunits exist in humans, Nav1.1-Nav1.9. The Nav1.x subfamily can be pharmacologically subdivided into tetrodotoxin (TTX)-sensitive or TTX-resistant. Nav1.7, (also named as PN1, SCN9A or hNE) is TTX-sensitive and is primarily expressed in peripheral sympathetic and sensory neurons. Nav1.7 accumulates at nerve fiber endings and amplifies small sub-threshold depolarizations and acts as a threshold channel that regulates excitability.
Nav1.7 function is implicated in various pain states, including acute, inflammatory and/or neuropathic pain. In man, gain of function mutations of Nav1.7 have been linked to primary erythermalgia (PE), a disease characterized by burning pain and inflammation of the extremities (Yang et al., J Med Genet 41:171-4, 2004), and paroxysmal extreme pain disorder (PEPD) (Fertleman et al., Neuron 52:767-74, 2006). Consistent with this observation, non-selective sodium channel blockers lidocaine, mexiletine and carbamazepine can provide symptomatic relief in these painful disorders (Legroux-Crespel et al., Ann Dermatol Venereol 130:429-33, 2003; Fertleman et al., Neuron 52:767-74, 2006).
Loss-of-function mutations of SNC9A in humans cause congenital indifference to pain (CIP), a rare autosomal recessive disorder characterized by a complete indifference or insensitivity to painful stimuli (Cox et al., Nature 444:894-8, 2006; Goldberg et al, Clin Genet 71:311-9, 2007; Ahmad et al., Hum Mol Genet 16:2114-21, 2007).
Single nucleotide polymorphisms in the coding region of SCN9A have been associated with increased nociceptor excitability and pain sensitivity. For example, a polymorphism rs6746030 resulting in R1150W substitution in human Nav1.7 has been associated with osteoarthritis pain, lumbar discectomy pain, phantom pain, and pancreatitis pain (Reimann et al., Proc Natl Acad Sci USA 107:5148-53, 2010). DRG neurons expressing the R1150W Nav1.7 display increased firing frequency in response to depolarization (Estacion et al., Ann Neurol 66:862-6, 2009). A disabling form of fibromyalgia has been associated with SCN9A sodium channel polymorphism rs6754031, indicating that some patients with severe fibromyalgia may have a dorsal root ganglia sodium channelopathy (Vargas-Alarcon et al., BMC Musculoskelet Disord 13:23, 2012).
In mice, deletion of the SCN9A gene in nociceptive neurons lead to reduction in mechanical and thermal pain thresholds and reduction or abolition of inflammatory pain responses (Nassar et al., Proc Natl Acad Sci USA 101:12706-11, 2004). Ablating Nav1.7 gene expression in all sensory neurons abolished mechanical pain, inflammatory pain and reflex withdrawal responses to heat. Deleting SCN9A in both sensory and sympathetic neurons abolished mechanical, thermal and neuropathic pain, and recapitulated the pain-free phenotype seen in humans with SCN9A loss-of-function mutations (Minett et al., Nat Commun 3:791, 2012). Nav1.7 inhibitors or blockers may therefore be useful in the treatment of a wide range of pain associated with various disorders.
Spider venoms are known to contain a large number of sodium channel blocking peptides, including Huwentoxin-IV (HwTx-IV) (Peng et al., J Biol Chem 277:47564-71, 2002), Protoxin-I, Protoxin-II (Middleton et al., Biochemistry 41:14734-47, 2002) and Phrixotoxin-III (Bosmans et al., Mol Pharmacol 69:419-29, 2006). Huwentoxin-IV (HWTx-IV), from the Chinese bird spider Ornithoctonus huwena, is a potent blocker of Nav1.7 and other TTX-sensitive voltage-gated sodium channels and likely functions as a gating modifier by trapping the voltage sensor of domain II in an inward, closed conformation (Xiao et al., J Biol Chem 283:27300-13, 2008). Protoxin-II, due to its favorable potency and selectivity profile, has been the subject of various in vivo studies aimed at demonstrating analgesia, none of which have reported success without disrupting the perineurium. Only through subversion of the blood-nerve barrier via desheathing of cutaneous nerves (Schmalhofer et al., Mol Pharm 74:1476-1484, 2008) or perineurial injection of hypertonic saline leading to down-regulation of tight junction protein claudin-1 (Hackel et. al., PNAS 109:29 E2018-27, 2012) was any efficacy observed for Protoxin-II. There is a need for identification of additional Nav1.7 blockers for treatment of a wide range of pain indications. In particular, there is a need for new Nav1.7 blockers with selectivity for Nav1.7 over other VGSC isoforms.